1. Introduction
The invention relates to radiation sensitive compositions such as photoresists that provide relief images of enhanced resolution. More particularly, the invention relates to compositions and methods that comprise novel means for treating photoacid-generating compositions to provide relief images of enhanced resolution and to control diffusion of photogenerated acid.
2. Background Art
Photoresists are used to form photosensitive films used for transfer of an image to a substrate. After a coating of a photoresist is formed on a substrate, the coating is selectively exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the resist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of the substrate.
A photoresist can be either positive-acting or negative-acting. For most negative photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the resist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
Following development of a photoresist coating, portions of the substrate bared by development may be altered such as by chemical etching or plating. The historical background, types and processing of conventional photoresists are described by DeForest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975, and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, 1988, both incorporated herein for their teaching of photoresist compositions and methods of making and using the same.
Most commercial photoresist formulations, both positive and negative, comprise a film forming resin binder and a radiation sensitive component. Many of the film forming binders in use are phenolic resins. For example, most positive acting photoresists currently in commercial use comprise a novolak resin and a naphthoquinone diazide sulfonic acid ester photoactive compound where the novolak resin is the reaction product of formaldehyde and a phenol. Examples of such photoresists are disclosed in U.S. Pat. Nos. 4,377,631 and 4,404,272, both incorporated herein by reference. Another class of positive acting photoresists comprise a poly(vinylphenol) and a naphthoquinone diazide acid ester. Examples of these photoresists are disclosed in U.S. Pat. Nos. 3,869,292 and 4,439,516, both incorporated herein by reference.
Many negative acting photoresists also utilize phenolic resins as the film-forming component of the resist. For example, photoresist compositions of particular utility in high resolution deep-UV lithography have been developed based on the use of a photoacid generator sensitive to selective wavelengths of radiation, a crosslinking agent, and a phenolic, acid-hardening, polymeric binder. In these systems, radiation is used to cleave the photoacid generator, thus creating a strongly acidic environment. Upon subsequent heating (a processing step referred to as the "post exposure bake"), the generated acid activates the crosslinking agent to react with the phenolic binder and thereby form a base insoluble negative image (negative-tone resist). The acid acts as a catalyst for the crosslinking, i.e., there are many crosslinking events per unit of acid generated in the film. Resists that rely on acid catalysts, such as these acid-hardening resists, have been classified generally as "chemically amplified photoresists".
In addition to catalyzed crosslinking, other chemically amplified mechanisms are known, for example, catalyzed deprotection. Exemplary of such a system is a positive-tone resist comprising a phenolic resin, a radiation sensitive component which generates acid upon irradiation, and a dissolution inhibitor which is not photosensitive itself, but is chemically decomposed in an acid-catalyzed deprotection reaction. As with the above described negative-acting system, the acid catalyst is catalytic, inducing a series of deprotection reactions upon heating during the post exposure bake.
More specifically, a deprotection process can be represented by the following equations:
Acid Generation Reaction EQU AG+hv.fwdarw.AH+G.fwdarw.A.sup.- +H.sup.+ +G
Deprotection Reaction EQU H.sup.+ +Poly-O-p.fwdarw.Poly-OH+H.sup.+
In the above reactions, the acid-generator (AG) molecule is converted to a strong acid (AH) upon absorption of a photon (hv), i.e., upon exposure to activating radiation. The acid proton affects the desired deprotection reaction of the protected polymer (Poly-O-p, where Poly-O is a hydroxyl-substituted polymer and p is a protecting group) to provide the deprotected polymer (Poly-OH) at a rate which is a function of the acid concentration [H.sup.+ ], temperature, diffusion rate of the acid in the polymer matrix and the process environment. A crosslinking mechanism operates similarly, the acid proton affecting the reaction between the crosslinker and the reactive polymer of the composition.
Adequate resolution of a patterned image generally requires that the radiation generated acid concentration, [H.sup.+ ], remain substantially constant within the exposed regions of a layer of the composition. The exposure process defines the latent image by transferring information to the resist coating layer by means of the phototool and the exposure radiation. This information is stored in the resist as photogenerated acid. Any loss of this information (i.e., acid) into unexposed regions of the resist, or into the substrate or environment can reduce the resolution of the transferred image.
In chemically amplified resists, it is generally important to have sufficient diffusion of photogenerated acid so that enough catalytic conversions occur to provide required photospeed. On the other hand and as discussed above, it is also important to limit diffusion of photogenerated acid to avoid information loss from exposed regions of a coating layer. Striking a satisfactory balance between these two objectives, however, can be difficult. Photogenerated acid often tends to diffuse into unexposed regions of a photoresist coating layer, or into the environment or the substrate. Upon subsequent heating during the post exposure bake, the acid-catalyzed reaction will occur in those unexposed regions where the acid has diffused, compromising resolution of the patterned image.
Methods for controlling diffusion of acid through an exposed photoresist layer have included redesign of the polymer matrix to provide large molecules to slow movement of the photogenerated proton, or to incorporate large molecules of photoacid compounds which generate large acid molecules. The use of large molecules has the disadvantage of decreasing the number of catalytic cycles for the chemically amplified process, thus decreasing the sensitivity of the resist.
Another method to control acid diffusion has been to limit the period between exposure and the post exposure bake. This can require design of relatively sophisticated and more costly interchanges between the phototool and baking apparatus.